I believe the newest version of iOS you can run on the original iPad is 5.1. Do you have the original iPad? Or iPad 2?
To update iOS to the newer version, connect your iPad to a computer with iTunes. I haven't used anything running iOS 4 in quite a while, so I'm not 100% sure, but you might also be able to update via WiFi... Look in your settings, and see if it lists "software update" under general settings.